?? If I read you correctly, it's booting from an empty drive. I'm missing something ??
Some background would be helpful. Is this a new machine, or did it run for years with another installation of the OS? Are these two brand-new hard drives, or were they used before, and was the one that you intend to use for storage bootable?
The simplest answer may be to go into BIOS and set your boot order, but I'm not sure exactly what you are asking here. In the simplest case, the BIOS has settings for which disk to select during the bot process.
In the most complex case, you had a running OS on the drive that you intend to use for storage, and both drives were running when you installed the OS on the other drive. In that case, the boot starts from the boot sector on the "storage" drive and then moves to the OS install on the OS drive but, as I said, this is the most complex case.